The specific surface is a physical property of pulps which is gaining more and more attention, particularly where mechanical pulps are concerned. The property is generally connected with the degree of surface development of individual fibres resulting from beating or refining (in a refiner), hence its importance in pulps which are made by mechanical means or which, if chemically made, are subjected to beating. It is defined as the total surface per unit weight of a pulp and it can be measured, of course indirectly, e.g. by the method described in the paper by A. A. Robertson and S. G. Mason (Pulp and Paper Magazine of Canada, December 1949, p.103-110).
While the average specific surface of a mechanical pulp is of interest per se for the characterization of mechanical pulps and for the development of on-line controls in the production of such pulps, more recently attention has been directed to the fractional distribution by weight of fibre specific surface of such pulps i.e., obtained by the fractionating of such pulps into several fractions, in increasing or decreasing order of values of specific surface, and the measuring of the specific surface of the respective fractions. In co-pending United States patent application No. 747,878 filed Dec. 6, 1976, now abandoned, by the same inventors, a process is described for reducing the linting propensity of a mechanical pulp, in which process the fraction or fractions of the pulp below a certain specific surface are subjected to additional mechanical working. Knoweldge of the fractional distribution by weight of fibre specific surface in such a pulp is, of course, of great help in deciding how big a fraction of the pulp should be thus reworked.